The present invention concerns the linearization of amplifiers, in particular the linearization of transmitter amplifiers used in transmitter equipment of radio communication systems, in particular mobile radio communication systems.
Known linearization techniques include those based on pre-correction applied to the signals to be amplified to compensate non-linear distortion caused by amplifying the signals.
Linearization techniques based on pre-correction require a knowledge of the non-linearities of the amplifier which is to be linearized. They can be determined once and for all, before the amplifier goes into service, or after the amplifier goes into service, by transmitting test signals, referred to hereinafter as linearization signals, when this is deemed to be necessary and analyzing the corresponding amplified linearization signals to determine the non-linearities of the amplifier and applying corresponding pre-correction to the wanted signals to be amplified by the amplifier.
A problem arises if the transmitter equipment includes a plurality of transmitters that are not totally decoupled from each other.
This situation arises if the capacity of the transmitter equipment is increased by using a plurality of transmitters which transmit in different frequency bands and are connected to the same antenna via a coupler or separately connected to different antennas. As the transmitters are not perfectly isolated from each other, either because the coupler used is not an ideal device or simply because of the geographical proximity of the various antennas, linearization signals transmitted by different transmitters can interfere with each other. The non-linearities of each of the transmitter amplifiers cannot be determined accurately in this situation.
To solve this problem it is possible to isolate the transmitters more effectively from each other by using filters with very high attenuation. Such filters are costly and bulky, however. Such isolation can also be very difficult, or even impossible, to achieve in the case of transmitters operating in nearby frequency bands, because excessively high selectivity is then required.
Another solution would be to use a coupler of better quality, but the same cost disadvantage is then incurred as in the previous solution, or to move the antennas a sufficient distance apart, but this is not a realistic solution.
The essential aim of the present invention is to provide a solution to this problem which avoids these drawbacks and is simple to put into practise.
The present invention applies to any system, whether of the multiple access type or not, and in the case of a multiple access system irrespective of the mode of multiple access, including TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access) and CDMA (Code Division Multiple Access).
Accordingly, the present invention essentially consists of a device for linearizing transmitter amplifiers for transmitter equipment including a plurality of transmitters each including a transmitter amplifier and not totally decoupled from each other, said device including means for controlling the transmission of linearization signals relative to each amplifier and means for analyzing the corresponding amplified linearization signals to determine the non-linearities of said amplifier and applying to wanted signals a pre-correction to compensate said non-linearities, said device being essentially characterized in that it includes central control means for commanding the transmission of linearization signals relating to different transmitter amplifiers at different times.